1. Field of the Invention
This invention relates generally to heaters. More specifically, the present invention relates to a linear heater which can be advantageously used in an office automation apparatus such as a photocopier or electrographic printer for fixing images on a paper sheet for example.
2. Description of the Prior Art
Various types of linear heaters are known for fixing images (deposited toner) on a paper sheet in photocopiers or electrographic printers (e.g. laser beam printer). Typical examples include a lamp heater and a roller heater.
However, the lamp heater and roller heater are equally disadvantageous in that there is a limitation in reducing size (thickness) and cost. Further, the lamp heater is easily damaged due to the nature of material, whereas the roller heater has a complicated structure due to the necessity of incorporating plural heating elements within the roller.
To eliminate the problems of the conventional heaters, U.S. Pat. No. 5,068,517 to Tsuyuki et al (Patented: Nov. 26, 1991; Filed: Aug. 22, 1989) proposes a strip heater which comprises an elongate insulating substrate having a surface provided with a printed resistor strip. Both ends of the resistor strip are enlarged and coated with silver for connection to a power source. The resistor strip, which is made of e.g. silver-palladium alloy, generates heat when a current is passed therethrough. The resistor strip is covered by a glass layer to provide smooth contact with a paper sheet.
Obviously, the strip heater of the above U.S. patent is very simple in arrangement. Further, the strip heater can be made very thin by reducing the thickness of the substrate. However, the strip heater is still disadvantageous in the following points.
First, since the resistor strip is continuous, it becomes inoperative even if it is broken or disconnected only at one portion thereof. Thus, in such an event, the strip heater as a whole must be replaced.
Secondly, the enlarged ends of the resistor strip, which are coated with silver, are the portions where heat dissipation occurs most easily. Thus, if the resistor strip is made to have a constant width over the entire length thereof, an uneven temperature distribution will result in which the surface temperature of the resistor strip is lower near the enlarged ends than at the center. This problem itself can be solved if the width of the resistor strip is made to reduce progressively toward the enlarged ends, as taught in the above U.S. patent. However, such a solution gives rise to a new problem that the narrower end portions of the resistor strip are more easily broken because, in spite of the reduced width, the narrower end portions generate a greater amount of heat than the central portion.